IS 2071 ( Part.1 ) : 1993 IEC Pub eO-1 ( 1989 ) ( Reaffirmed 2004 ) mT 1 RmTW Pf;zurrw@ d7 rl~wJTw?HTq ( FT g;r*@fq ) Indian Standard HIGH VOLTAGE TEST TECHNIQUES PART 1 GENERAL ( DEFINITIONS AND TEST REQUIREMENTS Second Revision ) First Reprint NOVEMBER 1996 UDC 621.317-32-027-3 @ BIS 199'3 BUREAU OF MANAK BHAVAN, INDIAN STANDARDS ZAFAR MARG 9 BAHADUR SHAH NEW DELHI 110002 October 1993 Price Rs. 22o.s IS 2071 ( Part 1 ) : 1993 IEC Pub 60-l ( 1989 ) CONTENTS Page National Foreword .......................................................................... Section'l: 1 2 General ................................................ 1 3 scope.. Object ................................................... ..................................................... 3 3 Sectiou 2: General Definitions ......................................... 4 4 4 4 4 5 5 5 Impulses ................................................... 3.1 Lightning and switching impulses ..... ......................... Characteristics related to disruptive discharge and test voltages .................... Disruptive discharge ........................................ 4.1 Characteristics of the test voltage ................................ 4.2 Disruptive discharge voltage of a test object .......................... 4.3 Statistical characteristics of disruptive discharge voltages .................. 4.4 Withstand voltage of a test object ................................ 4.5 Assured disruptive discharge voltage of a test object ..................... 4.6 Classification of insulation in test objects ................................ External insulation ......................................... 5.1 Internal insulation ......................................... 5.2 Self-restoring insulation ...................................... 5.3 Non-self-restoring insulation ................................... 5.4 6 6 6 6 7 7 7 Section 3: General Requirements Relating to Test Procedures and Test Objects ........................................... General requirements for test procedures ................................ ................................. g 8 8 General arrangement of the test object Drytests ................................................... 9 9 9 ................ 11 Wettests.....; ............................................. ................................... Standard wet test procedure 9.1 Traditional procedures for wet tests with alternating voltages 9.2 &324I71(Part1):1993 IEC Pob 60-l ( 1989 ) 10 Artificial pollution tests .......................................... 10.1 Preparation of test object ..................................... 10.2 Test procedures .......................................... ........................................ 10.3 Degree of pollution .......................................... Atmospheric conditions 11.1 Standard reference atmosphere .................................. 11.2 Atmospheric correction factors .................................. 11.3 Wet tests, tests under artificial pollution and combined tests ................. 11.4 Conflicting requirements for testing internal and external insulation ............ II.5 Measurement of humidity ..................................... 11 12 .12 13 14 14 14 1.5 11 16 16 Section 4: Tests with Direct Voltage 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Definitions for direct voltage tests .................................... .................... 12.1 Valueofthetestvohage 12.2 Ripple ................................................ Test voltage ................................................. ,13.1 Requirements for the test voltage ................................ .................................. 13.2 Generation of the test voltage 13.3 Measurement of the test voltage ................................. 13.4 Measurement of the test current ................................. ............................................... Testprocedures 14.1 Withstand voltage tests ...................................... 14.2 Disruptive discharge voltage tests ................................ 14.3 Assured disruptive discharge voltage tests ........................... 17 , ............... .17 17 13 17 17 17 18 19 19 19 20 14 20 Section 5: Tests with Alternating Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 15 Definitions for alternating voltage tests ................................. 15.1 Definitions for alternating voltage tests ............................. 15.2 Peak value ............................................ 15.3 R.M.S. value ........................................... Test Voltage ................................................. 16.1 Requirements for the test voltage ................................ .................................. 16.2 Generation of the test voltage 16.3 Measurement of the test voltage ................................. ............................................... Test procyures 17.1 Wrthstand voltage tests ...................................... 17.2 Disruptive discharge voltage tests ................................ 17.3 Assured disruptive discharge voltage tests ........................... 21 21 .21 .21 21 21 22 16 23 .24 17 24 24 25 (ii) IS 2071 ( Part 1) : 1993 IEC Pub 60-l ( 1989 ) Section 6: Tests with Lightning Impulse Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 18 Definitions for lightning impulse tests .................................. ............................... 18.1 Definitions of general applicability ....................... 18.2 Definitions applicable only to chopped impulses ........................................ 18.3 Voltage/time curves Test Voltage ................................................. 19.1 Standard lightning impulse .................................... 19.2 Tolerances .............................................. 19.3 Standard chopped lightning impulse ............................... .................................... 19.4 Special lightning impulses .................................. 19.5 Generation of the test voltage 19.6 Measurement of the test voltage and determination of impulse shape ............ ................. 19.7 Measurement of current during tests with impulse voltages Test Procedures ............................................... : .................................. 20.1 Withstand voltage tests ... 20.2 Procedures for assured discharge voltage tests ......................... 26 26 27 28 28 28 28 19 29 29 29 29 30 30 30 32 20 Section 7: Tests with Switching Impulses 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Definitions for switching impulse tests ................................. 21.1 Switching impulse ......................................... ..................................... 21.2 Value of the test voltage .......................................... 21.3 TimetopeakT, 21.4 Time to half-value Tz ....................................... 21.5 Time above 90% T,, ....................................... 21.6 Time to zero To ........................................... .................................... 21.7 Time to chopping T, ..... 21.8 Linearly rising impulse ...................................... Test voltage ................................................ 22.1 Standard switching impulse ................................... 22.2 Tolerances ............................................. 22.3 Special switching impulses .................................... .................................. 22.4 Generation of the test voltage 22.5 Measurement of test voltage and determination of impulse shape Test procedures 33 33 33 .33 33 .33 .33 34 34 .34 -34 .34 22 34 35 , .............. 35 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 Section 8: Tests with Impulse Current 24 . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Definitions for impulse current tests ................................... 24.1 Impulse current .......................................... 24.2 Value of the test current ..................................... 24.3 Front time Ti ........................................... 24.4 Virtual origin 0, ......................................... 24.5 Time to half-value T2 ....................................... 24.6 Duration of peak of a rectangular impulse current Td .................... 24.7 Total duration of a rectangular impulse current T, ....................... 36 .36 .36 .36 .36 36 36 37 (iii) IS 2071 ( Part I ) : 1993 IEC Pub 60-l ( 1989 ) 25 Test current ................................................. Standard impulse currents ..................................... 25.1 Tolerances ............................................. 25.2 Measurement of the test current ................................. 25.3 Measurement of voltage during tests with impulse current 25.4 37 37 .37 .................. 38 38 Section 9: Combined and Composite Tests 26 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Combined voltage tests .......................................... Value of the test voltage U .................................... 26.1 Time delay At ............................................ 26.2 Actual voltage shapes ....................................... 26.3 Arrangement of the test object .................................. 26.4 Atmospheric correction factors .................................. 26.5 39 39 39 39 40 40 27 Composite tests , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Appendix A: Statistical Treatment of Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 A.1 Classification of tests ........................................... A. 1.1 Class 1: Multiple-level tests ................................... A.1.2 Class 2: Up-and-down tests .................................... A.1.3 Class 3: Successive Discharge Tests ............................... ............................. A.2 Statistical Behaviour of Disruptive Discharge ............................. A.2.1 Confidence limits and statistical error 41 41 41 41 42 .42 .43 .43 .+I .44 .45 -45 A.3 Analysis of Test A.3.1 Treatment A.3.2 Treatment A.3.3 Treatment Results ......................................... of Results from Class 1 Tests ........................... of Results from Class 2 Tests ........................... of Results from Class 3 Tests ........................... A.4 Application of likelihood methods ................................... A.4.1 The likelihood function ...................................... A.4.2 Estimation of U,, and z ..................................... 46 Appendix B: Pollution Test Procedures . . . . . . . . ; . . . . . . . . . . . . . . . . . . . . .. . . . . . 47 47 47 B.1 Production of salt fog .......................................... B.l.l Preparation of salt solution .................. B.1.2 Details of spraying system .................................... ; ................ 47 47 47 48 4 49 B.2 Pre-deposition of pollution, coating and wetting procedure ..................... B-2.1 Preparation.of coating material ................................. ........................... B.2.2 Main characteristics of the inert materials .............................. B.2.3 Solid coating and wetting procedure B.3 Measurement of the degree of pollution ................................ B.3.1 Surface conductivity of the insulating surface ........................ 49 IS 2071 ( Part 1 ) : 1993 IEC Pub 60-l ( 1989 ) B-3.2 Equivalent amount of sodium chloride per square centimetre of the insulating surface (S.D.D. mg/cm2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Appendix C: Calibration of a Non-Approved Measurement Device with a Rod/Rod Gap . . . . . . 51 C.l General arrangement of a rod/rod gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Values , . . . . . . . . . . . . . . . . . . . . . . . . . . .......... ............. ................. C.2 Reference 51 51 C.3 Calibration Procedure ................. Figures ...................... ............. . . . . . . . . . . . . . . . . . 52 National Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...* 71 IS 2071 ( Part 1) : 1993 IEC Pub 60-l ( 1989 ) hdian Standard HIGHVOLTAGETESTTECHNIQUES PART 1 GENERAL DEFINITIONS AND TEST REQUIREMENTS ( Second Revision ) NATIONAL FOREWORD This Indian Standard which is identical with IEC 60-l : 1989 `High-voltage test techniques issued by the International Electrotechnical Part 1 : General definitions and test requirements', Commission ( IEC ) was adopted by the Bureau of Indian Standards on the recommendation of the High Voltage Engineering Sectional Committee ( ETD 19 ) and approved by the Electrotechnical Division Council. With a large number of high voltage and extra high voltage transmission system being constructed in this country, expensive electrical equipments are being put to service. It is necessary-to ensure that such equipments are capable to withstand the overvoltage normally encountered in service. This standard is intended to provide uniform methods of high voltage testing of electrical equipments. First Indian Standard on this subject IS 2070 was published in 1962 but it only covered method of impulse voltage testing. Subsequently IS 2071 series was brought out in the year 1974 which covered test procedure for dielectric tests with dc, ac and impulse voltages and impulse currents. This revision has been brought out to make it update and align it with the corresponding publication. This standard covers test requirements as well as test procedures. IBC With the publication of this standard IS 2071 ( Part 2 ) : 1974 will be withdrawn, because test procedures are also covered in Part I itself. A separate standard on measuring systems is under preparation and would replace IS 2071 ( Part 3 ) : 1976. The text of the IEC! standard has been approved as suitable for publication as Indian Standard without deviations. Certain conventions are however not identical to those used in Indian Attention is particularly drawn to the following: Standards. a) Wherever the words `International Standard' appear referring to this standard, they should be read as `Indian Standard'. b) Comma ( , ) has been used as a decimal marker, while in Indian Standards, the current practice is to use a point ( . ) as the decimal marker. CROSS REPERENCES In this Indian Standard, the following International respective place the following: International Standard for voltage measurement by means of sphere-gaps ( one sphere earthed ) IEC 60-3 : 1976 High voltage test techniques - Part 3 : Measuring devices IEC 60-4 : 1977 High voltage test techniques - Part 4 : Application guide for measuring devices IEC 52 : 1960 Recommendations Standards are referred to. Read in their Degree of Corresjondeke Equivalent Indian Standard IS 1876 : 1961 Method for voltage measurement by means of sphere gaps ( one sphere earthed ) IS 2071 ( Part 3 ) : 1976 Methods of high voltage testing : Part 3 Measuring devices IS 8690 : 1977 Application guide for measuring devices for high voltage testing I Equivalent Equivalent IS 2071 ( Part 1 ) : 1993 IEC Pub 60-l ( 1989 ) International Standard Indian Standard Degree of Correspondence IEC 270 : 1981 measurements Partial discharge IS 6209 : I982 Method for measurement discharge revision ) partial ( first Equivalent IEC! 507 : 1975 Artificial pollution tests on high-voltage insulators to be used on ac systems Technical Figure IS 8704 : 1982 Methods for artificial pollution tests on high voltage insulators for use on ac systems Equivalent has also taken note of the mistake in Fig. I and confirmed that in place of Figure 1 given in National Annex is to be followed. This is an obvious erro,r and IEC/TC 42 had also taken a decision similar in line and an amendment to IEC 60-l is expected soon. 1 given in main text, Committee Only English language text in the International this Indian Standard. Standard had been retained while adopting it in IS 2071 ( Part 1) : 1993 llx l'uh 60-l (lYS9) 1 Scope This standard is applicable to: dielectric tests with direct voltage; dielectric tests with alternating voltage; dielectric tests with impulse voltage; tests with impulse current; te'sts with combinations of the above. This standard is applicable only to tests on equipment having its highest voltage for equipment th, above 1 kV. This standard is not intended to be used for electromagnetic equipment. compatibility tests on electric or electronic 2 Object The object of this standard is: - to define terms of both general and specific applicability; to present general requirements regarding test objects and test procedures; to describe methods for generation and measurement of test voltages and currents; to describe test procedures; to describe methods for the evaluation of test results and to indicate criteria for acceptance or refusal. - Definitions and requirements concerning approved measuring devices and checking methods are given in IEC Publication 60-3: High Voltage Test Tech6iques - Measuring Devices. Alternative test procedures may be required to obtain reproducible and significant results. The choice of a suitable test procedure should be made by the relevant Technical Committee. 3 IS 2071 ( Part 1) : 1993 11x: Pub 60-l (19X9) Section 2: General Definitions 3 Impulses An impulse is an intentionally applied aperiodic transient voltage or current which usually rises rapidly to a peak value and then falls more slowly to zero. For special purposes, impulses having approximately approximately rectangular form are used. linearly rising fronts or transients of oscillating or The term "impulse" is to be distinguished from the term "surge" which refers to transients occurring in electrical equipment or networks in service. 3.1 Lightning and switching impulses A distinction is made between lightning and switching impulses on the basis of duration of the front. Impulses with front duration up to 20 ps are defined as lightning impulses and those with longer fronts are defined as switching impulses. Generally, switching impulses are also characterized lightning impulses. by total durations considerably longer than those of 4 4.1 Characteristics related to disruptive discharge and test voltages Disruptive discharge In this standard, the term "disruptive discharge" (sometimes referred to as "electrical breakdown") relates to phenomena associated with the failure of insulation under electrical stress, in which the discharge completely bridges the insulation under test, reducing the voltage between the electrodes practically to zero. It applies to electrical breakdown in solid, liquid and gaseous dielectrics and combinations of these. Non-sustained disruptive discharge in which the test object is momentarily bridged by a spark or arc may occur. During these events the voltage across the test object is momentarily reduced to zero or to a very small value. Depending on the characteristics of the test circuit and the test object, a recovery of dielectric strength may occur and may even permit the test voltage to reach a higher value. Such an event should be interpreted as a disruptive discharge unless otherwise specified by the relevant Technical Committee. Non-disruptive discharges such as those between intermediate electrodes or conductors may also occur without reduction of the test voltage to zero. Such an event should not be interpreted as a disruptive discharge unless so specified by the relevant Technical Committee. Some non-disruptive discharges are termed "partial discharges" and are dealt with in EC Publication Partial Discharge Measurements. The term "sparkover" is used when a disruptive discharge occurs in a gaseous or liquid medium. 270: IS 2071 (Part 11: 1993 IEC Pub 60-I (1989) The term "flashover" is used when a disruptive discharge occurs over the surface of a dielectric gaseous or liquid medium. The term "puncture" is used when a disruptive discharge occurs through a solid dielectric. in a A disruptive discharge in a solid dielectric produces permanent loss of dielectric strength; in a liquid or gaseous dielectric the loss may be only temporary. 4.2 Characteristics of the test voltage The characteristics of a test voltage are those characteristics specified in this standard for designating the different types of voltage excursion that define the test voltage. 4.2.1 Prospective characteristics of a test voltage The prospective characteristics of a test voltage causing disruptive discharge are the characteristics which would have been obtained if no disruptive discharge had occurred. When a prospective characteristic is used, this shall always be stated. I 4.2.2 Actual characteristics of a test voltage of a test voltage are those which occur during the test at the terminals of the The actual characteristics test object. 4.2.3 Value of the test voltage The value of the test voltage is defined in the relevant Clauses of the present standard. 4.3 Disruptive discharge voltage of a test object The disruptive discharge voltage of a test object is the value of the test voltage causing disruptive discharge, as specified, for the various tests, in the relevant Clauses of the present standard. 4.4 Statistical characteristics of disruptive discharge voltages Disruptive discharge voltages are subject to random variations and, usually, a number of observations must be made in order to obtain a statistically significant value of the voltage. The test procedures, described in the present standard, are generally based on statistical considerations. Information on the statistical evaluation of test results is given in Appendix A. 4.4.1 Disruptive discharge probability p of a test object The disruptive discharge probability p of a test object is the probability that one application of a certain prospective voltage value of a given shape will cause disruptive discharge in the test object. The parameter p may be expressed as a percentage or a fraction. 5 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 4.4.2 Withstand probability 9 of a test object The withstand probability q of a test object is the probability that one application of a certain prospective voltage value of a given shape does not cause a disruptive discharge on the test object. If the disruptive discharge probability is p. the withstand probability q is (1 -p). 4.4.3 50% disruptive discharge voltage U,, of a test object voltage value which has a 50% probability of The 50% disruptive discharge voltage is the prospective producing a disruptive discharge on the test object. 4.4.4 p% disruptive discharge voltage UP of a test object voltage value which has pl The p% disruptive discharge voltage of a test object is the prospective probability of producing a disruptive discharge on the test object. 4.4.5 Conventional deviation z of the disruptive discharge voltage of a test object The conventional deviation z of the disruptive discharge voltage of a test-object is the difference between its 50% and 16% disruptive discharge voltages. It is often expressed in per unit or percentage value, referred to the 50% disruptive discharge voltage. NOTE - If the disruptivedischarge probability function (see Appendix A) is close to a Gaussian function, z is correspondingly close to its standard deviation. 4.5 Withstand voltage of a test object voltage value which charactertzes the The withstand voltage of a test object is a specified prospective insulation of the object with regard to a withstand test. Unless otherwise specified, withstand voltages are referred to standard reference atmospheric conditions (see Clause 11.1). 4.6 Assured disruptive discharge voltage of a test object voltage value which The assured disruptive discharge voltage of a test object is a specified prospective characterizes its performance with regard to a disruptive discharge test. 5 Classification of insulation in test objects Insulation systems of apparatus and high voltage structures must basically be classified into self-restoring and non-self-restoring insulation and may consist of external and/or internal insulation. 5.1 External insulation External insulation is the air insulation and the exposed surfaces of solid insulation of the equipment, which are subject both to dielectric stresses and to the effects of atmospheric and other external conditions such as pollution, humidity and vermin. 6 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 5.2 Inrernal insulation Internal insulation comprises the internal solid, liquid or gaseous elements of the insulation of equipment, which are protected from the effects of atmospheric and other external conditions such as pollution, humidity and vermin. 5.3 Self-restoring insulation Self-restoring insulation is the insulation which completely recovers disruptive discharge caused by the application of a test voltage. 5.4 Non-self-restoring insulation its insulating properties after a Non-self-restoring insulation is insulation which loses its insulating properties, or does not recover them completely, after a disruptive discharge caused by the application of a test voltage. In high voltage apparatus, parts of both self-restoring and non-self-restoring insulation are always operating in combination and some parts may be degraded by repeated or continued voltage applications. The behaviour of the insulation in this respect shall be taken into account by the relevant Technical Committee when specifying the test procedures to be applied. NOTE - 7 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) Section 3: General Requirements Relating and Test Objects to Test Procedures 6 General requirements for test procedures The test procedures applicable to particular types of test objects, for example, the polarity to be used, the preferred order if both polarities are to be used, the number of applications and the interval between applications shall be specified by the relevant Technical Committee, having regard to such factors as: - the required accuracy of test results; the random nature of the observed phenomenon and any polarity dependence of the measured characteristics; the possibility of progressive deterioration with repeated voltage applications. - 7 General arrangement of the test object At the time of a test, the test object shall be complete in all essential details, and it should have been processed in the normal manner for similar equipment. The disruptive discharge characteristics of an object may be affected by its general arrangement (for example, by its clearance from other live or grounded structures, its height above ground level and the arrangement of its high voltage lead). The general arrangement should be specified by the relevant Technical Committee. A clearance to extraneous structures not less than 1.5 times the length of the shortest possible discharge path on the test object usually makes such proximity effects negligible. In wet or pollution tests, or wherever the voltage distribution along the test object and the electric field around its energized electrode are sufficiently independent of external influences, smaller clearances may be acceptable, provided that discharges do not occur to extraneous structures. In the case of a.c. or positive switching impulse tests above'750 kV (peak) the influence of an erttraneous structure may be considered as negligible if its distance from the energized electrode is also not less than the height of this electrode above the ground plane. A practical lower limit to this clearance is given in figure 1, as a function of the highest test voltage. A withstand {est may be acceptable when successfully performed with shorter distances to earthed objects. IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 8 Dry tests The test object shall be dry and clean. If not otherwise specified by the relevant Technical Committee, the test should be made at ambient temperature and the procedure for voltage application shall be as specified in the relevant Clauses of this standard. 9 Wet tests The preferred wet test procedure, described in 9.1. is intended to simulate the effect of natural rain on external insulation and is a revision of earlier test methods. It is recommended for tests with all types of test voltages and on all types of apparatus, but either of the alternative test methods given below are permitted if specified by the relevant Technical Committee. Two earlier test methods. not intended to simulate natural rain, are described in 9.2. They have been in use for many years for tests with alternating voltages on apparatus having U, up to 420 kV and many test data obtained by these methods exist. For a.c. apparatus of large dimensions, such as those having Um higher than 800 kV, no appropriate test procedure is available at present. wet The relevant Technical Committee shall specify the arrangement of the test object during the test procedure. 9.1 Standard wet test procedure The test object shall be sprayed with water of prescribed resistivity and temperature (see table 1) falling .on it as droplets (avoiding fog and mist) and directed so that the vertical and horizontal components of the spray intensity are approximately equal. These intensities are measured with a divided collecting vessel having openings of 100 cm* to 750 cm*, one horizontal and one vertical, the vertical opening facing the spray. The position of the test object relative to the vertical and horizontal rain components by the relevant Technical Committee. shall be specified In general, the reproducibility of wet test results is less than that for other high voltage discharge or withstand tests. To minimize the dispersion the following precautions shall be taken: - The collecting vessel shall be placed close to the test object, but avoiding the collection of drops or splashes from it. During the measuring period, it should be moved slowly over a sufficient area to average but not completely mask the effect of non-uniformities of the spray from individual nozzles. This measuring zone shall have a width equal to that of the test object and a maximum height of 1 m. For test objects between l m and 3 m in height, the individual measurements shall be made at the top, centre and bottom of the test object. Each measuring zone shall cover only one third of the height of the test object. - IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) - For test objects exceeding 3 m in height, the number of measuring zones shall be increased to cover the full height of the test object without overlapping. The above procedures shall be suitably adapted for test objects having large horizontal dimensions. The spread of results may be reduced if the test object is cleaned with a surface-active which has to be removed before the beginning of wetting. detergent - - - The spread of results may also be affected by local anomalous (high or low) precipitation rates. It is recommended to detect these by localized measurements and to improve the uniformity of the spray, if necessary. conditions The spray apparatus shall be adjusted to produce, within the specified tolerances, precipitation at the test object given in table 1. Any type and arrangement of nozzles meeting the requirements given in table 1 may be used. Examples of several nozzles which have been found satisfactory in practice are shown in figures 2a, 2b and 2c, together with typical performance data for each type. Greater spray distances may be obtained if the nozzles'are directed upward at an angle of about W-25' to the horizontal. Note that if the water pressure is increased above the recommended limits, the water jets may break up prematurely and cause an unsatisfactory spray at the test object. Table 1 - Precipitation conditions for standard procedure Average precipitation rate of all measurements - vertical component - horizontal component Limits for any individual measurement and for each component Tempemture of water Resistivity of water Rm loo* 15 mm/mill mm/min I,Oto20 I.0 to 2.0 mm/mm H.5 from average The water temperature and resistivity shall be measured on a sample collected immediately before the water reaches the test object. They may also be measured at other locations (e.g., in a storage reservoir) provided that a check ensures that no significant change occurs by the time the water reaches the test object. The test object shall be pre-wetted initially for at least 15 min under the above specified conditions and these conditions shall remain within the specified tolerances throughout the test which should be performed without interrupting the wetting. The pre-wetting time shall not include the time needed for adjusting the spray. It is also possible to perform an initial pre-wetting by unconditioned mains water for 15 min. followed without interruption of the spray by a second pre-wetting for at least 2 min before the test begins, 10 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) using water with all the correct precipitation starting the test. Unless otherwise the same as that if not otherwise mended that one specified specified specified. flashover conditions, which should be measured immediately before by the relevant Technical Committee, the test procedure for for the corresponding dry tests. The test duration for an a.c. In general, for alternating and direct voltage wet withstand should be permitted provided that in a repeat test no further wet tests shall. be test shall be 60 s. tests, it is recomflashover occurs. 9.2 Traditional procedures for wet tests with alternating voltages For alternating voltage tests, two other procedures are also in use, details of which are given in table 2. They differ from the standard procedure, 9.1, primarily in that the precipitation rates are higher and that the minimum pre-wetting time is only 1 m. Only the vertical component of the spray is specified; determination of the horizontal component is replaced by a visual estimate of the spray angle which should be approximately 45' at the test object. Table 2 - Precipitation conditions for traditional procedures with alternating voltages Characteristics European practice Practice in U.S.A. Average precipitation rate of all measurements: vertical component mm/mm mm/min `C , 3 f 0.3 3 f 0.75 5f05 5 z!I1.25 Limits for any individual measurement Water temperature Water resistivity Ambient temperature f 15 loo+ 10 2a2b Zc 178 + 27 figure26 10 SZm Type of nozzle as shown in figures Duration of wet withstand test S 60 10 Artificial pollution tests Artificial pollution tests are intended to provide information on the behaviour of external insulation under conditions representative of pollution in service, although they do not necessarily simulate any particular service conditions. 11 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) The following specifications give some general guidance on artificial pollution testing. It is left to the relevant Technical Committee to introduce variations or to give more specific requirements for particular classes of apparatus. Such specific information is given in one instance by IEC 507. The effects of washing of insulators in service by natural rain is not taken into consideration specified procedures. 10.1 Preparation of test object in any of the Before testing for the first time, the metal parts of the test object, and any cement joints, may be painted with salt-water-resistant paint to ensure that corrosion products will not contaminate the insulating surfaces during a test. The test object should then be carefully cleaned by washing with tap water to which trisodium phosphate (Na,PO,) has been added and rinsed with clean tap water. It shall not subsequently be touched by hand. Usually the insulating surfaces can be considered sufficiently clean and free of grease or other contaminating material if large continuous wet areas are observed during wetting. It is left to the relevant Technical Committee to decide whether the test object should be tested in a vertical, horizontal or an inclined position. 10.2 Test procedures Artificial pollution tests involve application of the pollution and the simultaneous or subsequent application of voltage. Generally, only methods in which the test voltage is held constant for at least several minutes are recommended. Other methods in which the voltage is raised gradually to flashover are not proposed for standardization but may be used for special purposes. The pollution test may be made either to determine the maximum degree of pollution of the test object which allows a given test voltage to be withstood, or to determine the withstand voltage for a specified degree of pollution. For the purpose of comparing the results of several tests, or the performance of several test objects, the former procedure is preferable. Whichever test procedure is adopted, the number of measurements should be sufficient to obtain consistent average values, taking into account the statistical nature of the phenomenon. The number of tests required shall be specified by the relevant Technical Committee. The pollution tests fall into two categories, the salt-fog method and the pre-deposited pollution method. a) The salt-fog method The test object is placed in a special chamber which can be filled by a salt fog. The method for producing the fog is described in Appendix Bl. The ambient temperature in the chamber at the start of the test shall not be less than 5-C. nor greater than 3O'C and the test object and the salt water shall be in thermal equilibrium with the ambient temperature. The test object is thoroughly wetted with clean tap water. The salt-fog system, supplied by water of the prescribed salinity, is started when the test object is still wet and, simultaneously, the voltage is applied to the test object, raised rapidly to the specified value and kept constant during the specified time, usually 12 IS 2071'( Part 1) : 1993 IEC Pub 60-l (1989) 1 h, or until flashover occurs. This procedure is repeated several times. Before each procedure object is thoroughly washed with clean tap water to remove any trace of salt. the test For the salt-fog method, the minimum distance between any part of the test object and any earthed object other than the jets and the structure which supports the insulator shall be not less than 0.5 m per 100 kV of the test voltage and, in any case, not less than 2 m. If the test is intended to determine the maximum degree of salinity for a specified withstand voltage, the whole procedure must be repeated using various salinities. Pre-conditioning of the test object by a number of flashovers during the application of pollution is required before the real test begins. This pre-conditioning should be followed by a washing. b) The pre-deposited pollution method The test object is coated with a reasonably uniform layer of a conductive suspension and shall be permitted to dry. The ambient temperature in the test chamber at the start of the test should not be less than 5'C nor greater than 3o'C and the test object should be in thermal equilibrium with the ambient. The wetting shall be accomplished by means of a steam fog generator which provides a uniform fog distribution over the whole length and around the test object. The temperature of the fog in the vicinity of the test object shall not exceed 4O'C. To obtain the necessary wetting within a reasonable time, enough steam fog shall be introduced inside the test chamber. The steam generation rate shall be specified by the relevant Technical Committee. In one procedure the voltage is applied before the test object is wetted by the fog and continues until flashover or for about twice the time for the insulator to achieve its maximum conductivity. In another procedure, the test voltage is applied only when the conductivity has reached its maximum value, which should occur between 20 and 40 min from the start of fogging. The voltage shall be kept constant during the specified 15min test time or until flashover occurs. Examples of ,auita~ coating and wetting procedures and of the measurement of the surface resistivity are given in Appendix B. The procedure above may be repeated several times; before each test, the test object shall be washed, re-coated and allowed to dry. When the test is intended to determine the maximum degree of pollution for a specified withstand voltage, the coating, wetting and test procedures must be repeated using various suspension resistivities. The minimum distance between any part of the test object and any earthed cbject other than the structure which supports the test object shall be not less than 0.5 m per 100 kV of the test voltage. 10.3 Degree of pollution The degree of pollution of a test object is specified by the salinity (g/L) of the salt fog, by the surface conductivity (@I) or by the amount of salt (NaCl) per square centimetre of the insulating surface (gm/cm'). This latter is normally referred to as the Salt Deposit Density (S.D.D.). Information about these methods is given in Appendix B. 13 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) 11 Atmospheric conditions Standard reference atmosphere 11.1 The standard reference atmosphere is: temperature pressure absolute humidity t, = 2o'c bO= lO1,3 kPa (1013 mbar) fro= 11 g/m3 NOTE - A pressure of 101.3 kPa corresponds to the height of 760 mm in a mercury barometer at O'C. If the barometer height is H mm of mercury, the atmospheric pressure in kiiopascals is approximately: b= 0.1333HkPa Correction for temperature with respect to the height of the mercury column is considered to be negligible. 11.2 Atmospheric correction factors The disruptive discharge of external insulation depends upon the atmospheric conditions. Usually, the disruptive discharge voltage for a given path in air is increased by an increase in either air density or humidity. However, when the relative humidity exceeds about 80%. the disruptive discharge voltage becomes irregular, especially when the disruptive discharge occurs over an insulating surface. By applying correction factors, a disruptive discharge voltage measured in given test conditions (temperature t, pressure b, humidity h) may be converted to the value which would have been obtained under the standard reference atmospheric conditions (t, b, h,). Conversely, a test voltage specified for given reference conditions can be converted into the equivalent value under the test conditions. The disruptive discharge voltage is proportional to the atmospheric correction factor K,, that results from the product of two correction factors: the air density correction factor k, (see 11.2.1); the humidity correction factor b (see 11.2.2). If not otherwise specified by the relevant Technical Committee, the voltage U to be applied during a test on external insulation is determined by multiplying the specified test voltage lJ, by K; Similarly, measured disruptive discharge voltages U are corrected to U, corresponding ence atmosphere by dividing by K,: U, = UIK, to standard refer- 14 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) The test report shall always contain the actual atmospheric conditions during the test and the correction factors applied. 11.2.1 Air density correction factor k, The air density correction factor k, depends on the relative air density 6 and can be generally expressed as: k, = 6" where m is an exponent given in 11.2.3. When the temperatures t and to are expressed in degrees Celsius and the atmospheric pressures b and bO in the same units (kilopascals or millibars), the relative air density is: 6_ are expressed b 273% --bO 273+r 11.2.2 Humidity correction factor k2 The humidity correction factor may be expressed as: where w is an exponent given in 11.2.3 and k is a that, for practical purposes, may be approximately h, to the relative air density, 6, using the curves humidity corrections are still under consideration, limits. 11.2.3 Exponents m and w parameter that depends on the type of test voltage and obtained as a function of the ratio of absolute humidity, of figure 3. For values of h / 6 in excess of 15 g/m' and the curves in figure 3 may be regarded as upper As the correction factors depend on the type of predischarges, considering the parameter: this fact can be taken into account by where U, is the 50% disruptive-discharge voltage (measured or estimated) at the actual atmospheric conditions, in kilovolts, L the minimum discharge path in metres, with the actual values for the relative air density 6 and for the parameter k. In the case of a withstand test where an estimate of the 50% disruptive discharge voltage is not available, U, can be assumed to be 1.1 times the test voltage. The exponents m and w are still under consideration. 11.3 Approximate values are given in figure 4. Wet tests. tests under artificial pollution and combined tests No humidity correction shall be applied for wet tests or for tests with artificial pollution. The question of density correction during such tests is under consideration. For combined tests see Clause 26.5. 15 1S 2071 (Pat-t 1) : 1993 IEC Pub 60-l (1989) 11.4 Conflicting requirements for testing internal and external insulation While withstand levels are specified under standard atmospheric conditions, cases will arise where the application of atmospheric corrections (due to laboratory altitude or to extreme climatic conditions) results in the withstand level for internal insulation appreciably in excess of that for the associated external insulation. In such cases measures to enhance the withstand level of the external insulation must be adopted to permit application of the correct test voltage to the internal insulation. These measures include immersion of the external insulation in liquids or compressed gases and should be specified by the relevant Technical Committee with reference to the requirements of particular classes of apparatus. In those cases where the test voltage of the external insulation is higher than that of the internal insulation, the external insulation can only be correctly tested when the internal insulation is especially designed with increased strength. If not, the internal insulation should be tested with the rated value and the external insulation be tested by means of dummies unless the relevant Technical Committee states otherwise, in which case it shall specify the test procedure to be used. 11.5 Measurement of humidity The humidity shall be determined preferably with the meter measuring directly the absolute humidity with an absolute error not larger than 1 g/m'. Measurement of relative humidity associated with the temperature measurements also allows determination of the absolute humidity and can be used provided that the accuracy of the absolute humidity determination in this case is the same as required above. NOTE - This measurement may also be made by means of a ventilated wet and dry bulb hygrometer. The absolute humidity as a function of the thermometer readings is determined from figure 5 which also permits determination of the relative humidity. It is important to provide adequate air flow in order to reach a steady state and to read the thermometers carefully in order to avoid excessive errors in the determination of the humidity. 16 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) Section 4: Tests with Direct Voltage 12 12.1 Definitions for direct voltage tests Value of the test voltage mean value. The value of the test voltage is defined as its arithmetic 12.2 Ripple Ripple is the periodic deviation from the arithmetic mean value of the voltage. The amplitude of the ripple is defined as half the difference between the maximum and minimum values. The ripple factor is the ratio of the ripple amplitude to the arithmetic mean value. 13 Test voltage Requirements for the test voltage Voltage shape 13.1 13.1-l The test voltage, as applied to the test object, should be a direct voltage with not more than 3% ripple factor, unless otherwise specified by the relevant Technical Committee. Note that the ripple factor may be affected by the presence of the test object and by the test conditions, especially in wet tests and in tests under artificial pollution. 13.1.2 Tolerances For test durations not exceeding 60 s, the measured values of the test voltage shall be maintained within fl% of the specified level throughout the test. For test durations exceeding 60 s, the measured value of the test voltage shall be maintained within +3% of the specified level throughout the test. NOTE - It is emphasized that the toleiance constitutes the permitted difference between the specified value and that actually measured. This difference should be distinguished from the measuring error which is the difference between the measured value and the true value. 13.2 Generation of the test voltage The test voltage is generally obtained by means of rectifiers, though sometimes electrostatic generators are employed. The requirements to be met by the test voltage source depend considerably upon the type of apparatus which 1s to be tested and on the test conditions. These requirements are determined mainly by the value and nature of the test current to be supplied, the important constituents of which are indicated in 13.4. The source characteristics should be such as to permit charging of the capacitance of the test object in a reasonably short time. In the case of objects having high capacitance, charging times of several minutes must sometimes be accepted. The source, including its storage capacitance, should also be adequate to supply the leakage and absorption currents and any internal and external non-disruptive discharge currents 17 1S 207 1 ( Part 1) : 1993 IIX Pub 60-l (19X9) without voltage drops exceeding 10%. In tests on internal insulation, these currents are usually small, but when testing wet insulators, leakage currents of the order of some tens of milliamperes or pre-discharge pulses of the order of lO-2 C may occasionally be encountered. Source parameters for D.C. pollution tests are under investigation. 13.3 13.3.1 Measurement of the test voltage Measurement with devices approved under IEC Publication 60-3: High Voltage Test Techniques - Measuring Devices The measurement of the arithmetic mean value, the maximum value, the ripple factor and any transient drop in the test voltage should, in general, be made with devices which have passed the approval procedure referred to in IEC Publication 60-3. Attention is drawn to the requirements on response characteristics of devices used for measuring ripple, transients or voltage stability. 13.3.2 Cafibraripn of a non-approved measuring device with an approved measuring device The procedure usually consists of establishing a relationship between the display of some device related to the test voltage and a measurement of the same voltage performed in accordance with 13.3.1, with a sphere-gap, used in accordance with IEC Publication 52, or with a rod/rod gap, used in accordance with 13.3.3. This relationship may be dependent on the presence of the test object, the sphere-gap or rod/rod gap, on the precipitation in wet tests, etc. Hence, it is important that these conditions are the same during the calibration and the actual test, except that, during the test, the sphere-gap or rod/rod gap shall be opened sufficiently to,prevent sparkover. The relationship between the supply voltage and the output voltage may be insufficiently stable for measuring purposes. Attention is drawn to the precautions necessary when using a sphere-gap under direct voltages, due to the occurrence of flashovers at lower voltage values predominantly resulting from the presence of microscopic fibrous particles. A series of'voltage applications shall be made and the highest voltage value is taken as the true measure. NOTE 1 - The problem of fibrous particles can be overcome by providing an air flow of not less than 3 m/s through the gap. NOTE 2 In the presence of ripple, sphere-gaps do not measure the arithmetic mean value of the voltage. The calibration is preferably made at or near 100% of the test voltage, but for tests on objects with non-self-restoring insulation, extrapolation may be made from a value not lower than 50% of this voltage. Extrapolation may be unsatisfactory if the current in the test circuit varies non-linearly with the applied voltage. 18 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) The rod/rod gap as an approved measuring device 13.3.3 A rod/rod gap with dimensions as given in Appendix C and used in accordance with this Appendix is an approved measuring device for measuring direct voltages. 13.4 Measurement of the test current When measurements of current through the test object are made, a number of separate components may be recognized. These differ from each other by several orders of magnitude for the same test object and test voltage. They are: - the capacitance current, due to the initial application of the test voltage and to any ripple or other fluctuations imposed on it; the dielectric absorption current, due to slow charge displacements within the insulation and persisting for periods of a few seconds up to several hours. This process is partially reversible. currents of the opposite polarity being observed when the test object is discharged and shortcircuited; the continuous leakage current, which is the final steady direct current attained at constant applied voltage after the above components have decayed to zero; partial discharge currents. - - - Measurement of the first three components necessitates the use of instruments covering a wide range of current magnitudes. It is important to ensure that the instrument, or the measurement of any one component of the current, is not adversely affected by the other components. Information concerning the condition of the insulation may sometimes be obtained by observing current variations with respect to time, during non-destructive tests. The relative magnitude and the importance of each component of current depend on the type and the condition of the test object, the purpose for which the test is being made and the duration of the test. Accordingly, the measurement procedures should be specified by the relevant Technical Committee, especially when it is required to distinguish a particular component. Measurements of partial discharge pulse currents are made with special instruments which are dealt with in IEC Publication 270 (1981): Partial Discharge Measurements. NOTE - Attention should be paid to the possible value of current flowing in the case of a disruptive discharge, that could destroy a current meter if not adequately protected. 14 14.1 Test procedures Withstand voltage tests The voltage shall be applied to the test object starting overvoltage due to switching transients. It should be instruments, but not so slowlyas to cause unnecessary the test voltage U. These requirements are in general at a value sufficiently low to prevent any effect of raised sufficiently slowly to permit reading of the prolongation of stressing of the test object near to met if the rate of rise is about 2% of U per second 19 IS 2071(l'art1):1993 IEC Pub 60-l(1989) when the applied voltage is above 75% of U. It shall be maintained for the specified time and then reduced by discharging the circuit capacitance, including that of the test object, through a suitable resistor. The test duration shall be specified by the relevant Technical Committee taking into consideration that the time to reach the steady-state voltage distribution depends on the resistances and capacitances of the test object components. When not otherwise specified by the relevant Technical Committee, the duration of a withstand test shall be 60 s. The polarity of the voltage or the order in which voltages of each polarity are applied, and any required deviation from the above specifications, shall be specified by the relevant Technical Committee. The requirements 14.2 of the test are satisfied if no disruptive discharge occurs on the test object. Disruptive discharge voltage tests The voltage shall be applied and raised continuously until a disruptive discharge occurs on the test object. The value of the voltage reached at the instant of the disruptive discharge shall be recorded. The relevant Technical Committee shall specify the voltage rate of rise, the number of voltage applications and the procedure for evaluating the test results (see Appendix A). 111.3 Assured disruptive discharge voltage tests The voltage shall be applied and raised continuously until a disruptive discharge occurs on the test object. The value of the test voltage reached at the instant of the disruptive discharge shall be recorded. The requirements of the test are generally satisfied if this voltage does not exceed the assured disruptive discharge voltage on a specified number of voltage applications. The relevant Technical Committee shall specify the number of voltage applications and the voltage rate of rise. 20 IS 2071 ( Part 1) : 1993 JEC Pub 60-l (1989) Section 5: Tests with Alternafing Voltage 15 15.1 15.i.l Definitions for alternating voltage tests Definitions for alternating voltage tests Value of the test voltage The value of the test voltage is defined as its peak value divided by 6. NOTE - The relevant Technical Committee may require a measurement of the r.m.s. value of the tesf voltage instead of the peak value for cases where the r.m.s. value may be of importance, for instance, when thermal effects are involved. 15.2 Peak value oscillations, arising The peak value of an alternating voltage is the maximum value. Small high-frequency for instance from non-disruptive discharges shall, however, be disregarded. 15.3 R.M.S. value The r.m.s. value of an alternating voltage is the square root of the mean value of the square of the voltage values during a complete cycle. 16 16.1 16.1.1 Test Voltage Requirements for the test voltage Voltage waveshape The test voltage shall be an alternating voltage generally having a frequency in the range 45 to 65 Hz, normally referred to as power-frequency test voltage. Special tests may be required at frequencies considerably below or above this range, as specified by the relevant Technical Committee. The voltage waveshape shall approximate a sinusoid with both half-cycles closely alike. The results of a high voltage test are thought to be unaffected by small deviations from a sinusoid if the ratio of peak to r.m.s. values equals *within k5%. For some test circuits in common use greater deviations have to be accepted. Note that the test object, especially if it has non-linear impedance characteristics, may considerably affect the deviation from a sinusoid. NOTE It can generally be assumed that the above requirements on deviations from a sinusoid will be met if the r.m.s. value of the harmonics does not exceed 5% of the r.m.s. value of the fundamental. 21 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 16.1.2 Tokrances If not otherwise specified by the relevant Technical Committee the measured values of the test voltage shall be maintained 60 s the measured within +l% of the specified level throughout the test. For test durations excesding value of the test voltage shall be maintained within +-3% of the specified level throughout the test. It is emphasized that the tolerance constitutes the permitted difference between and that actually measured. This difference should be distinguished from the measuring difference between the measured value and the true value. NOTE the specified error which value is the 16.2 16.2.1 Generation of the lest voltage General requirements The test voltage is generally supplied from a step-up transformer. Alternatively, means of a series-resonant circuit. it may be generated by The voltage in the test circuit shall be stable enough to be practically unaffected by varying leakage currents. Non-disruptive discharges in the test object shall not reduce the test voltage to such an extent and for such a time that the measured disruptive discharge voltage of the test object is significantly affected. In the case of non-disruptive discharges. unless otherwise specified by the relevant Technical Committee, a withstand test is considered satisfactory when it can be shown that the peak value of the test voltage does not differ by more than 5% in successive periods and that the instantaneous voltage drop during a non-disruptive discharge does not exceed 20% of the peak voltage. The characteristics of the test circuit which are necessary to meet the above requirements depend on the type of test (dry, wet, etc.), the test voltage level and the test object behaviour. NOTE - Attention is drawn to the possibility that such non-disruptive discharges may cause large overswings of voltage between the terminals of the test object. This phenomenon may cause failure of the test object or of the testing transformer. A cure can usually be effected by changing the natural frequency of the voltage source or by introducing some attenuation into the system. 16.2.2. Requirements for the transformer test circuit In order to have the test voltage practically unaffected by varying leakage currents the short-circuit current, delivered by the transformer when the test object is short-circuited at the test voltage, should be large enough in comparison with the leakage currents at the supply frequency and in any case in respect of the following guiding criteria: for dry tests on small samples of solid insulation, insulating liquids or combinations of the two, a short-circuit current of the order of 0.1 A (r.m.s.) is suitable; for tests on external self-restoring insulation (insulators, disconnecting switch, etc.) a short-cirl &it current not less than 0.1 A (r.m.s.) for dry tests and 0.5 A (r.m.s.> for wet tests is suitable: however, for wet tests on objects having large dimensions that may lead to high leakage currents, a short-circuit current up to 1 A could be necessary. - NOTE Publication When the test circuit is supplied by a rotating generator. the transient short-circuit current (see IEC 34-4) should be considered. 22 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) The total capacitance of the test object and of any additional capacitor should be sufficient to ensure that the measured discharge voltage is unaffected by non-disruptive partial discharges or pre-discharges in the test object. A capacitance in the range from 0.5 to 1.0 nF is generally sufficient. NOTE -If capacitance any protective resistor external to the test transformer does not exceed 10 kC2, the effective of the transformer may be regarded as being in parallel with the test object. terminal For tests under artificial pollution, higher values of the short-circuit current, up to 15 A or more, are necessary (see IEC Publication 507); the testing plant should also comply with the two following conditions: - resistance/reactance ratio (R/X) equal to or higher than 0.1; current ratio not exceeding the interval 0,001 to 0.1. capacitive current/short-circuit The voltage stability could be verified by the direct recording of the voltage applied to the test object, by means of a suitable high voltage measuring system. 16.2.3 The series-resonant circuit The series-resonant circuit consists essentially of an inductor in series with a capacitive, test object or load and connected to a medium voltage power source. Alternatively it may consist of a capacitor in series with an inductive test object. By varying the circuit parameters or the supply frequency, the circuit can be tuned to'resonance, when a voltage considerably greater than that of the source and of substantially sinusoidal shape will be applied to the test object. The stability of the resonance conditions and of the test voltage depends on the constancy of the supply frequency and of the test circuit characteristics. When a di,scharge occurs, the source gives a relatively low current which limits the damage to the dielectric of the test object. The series-resonant circuit is especially useful when testing objects such as cables, capacitors or gasinsulated systems in which the leakage currents on the external insulation are very small in comparison with the capacitive currents through the test object or the energy to form a disruptive discharge is very small. A series-resonant.circuit is also useful for testing reactors. The circuit may be unsuitable for external insulation under wet or polluted conditions, unless the reqtiirements of 16.2.1 are satisfied. 16.3 16.3.1 Measurement of the test voltage Measurement with devices approved under IEC Publication 60-3 The measurement of the peak value, the r.m.s. value, the deviation from a sinusoid and the transient drops should in general be made with devices which have passed the approval procedures referred to in IEC Publication 60-3. Attention is drawn to the requirements travsient voltage drops. on response characteristics of the devices used for measuring 23 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 16.3.2 Calibration of a non-approved measuring device with an approved measuring device The procedure usually consists of establishing a relationship between the display of some device related to the test voltage and a measurement of the same voltage performed in accordance with 16.3.1 or with a sphere-gap used in accordance with lEC Publication 52. This relationship may be dependent on the presence of the test object and the sphere-gap, the precipitation in wet tests.. etc. Hence, it is important that these conditions are the same during the calibration and the actual test, except that, during the test, the sphere-gap may be opened sufficiently to prevent spa&over. The relationship between the supply voltage and the output voltage may not be sufficiently measuring purposes. stable for The calibration is preferably made at or near 100% of the test voltage, but for tests on objects with non-self-restoring insulation, extrapolation may be made from a value not lower than 50% of this voltage. Extrapolation may be unsatisfactory if the current in the test circuit varies non-linearly with the applied voltage, or if any changes occur in the voltage shape or frequency between the calibration and the test voltage levels. 17 17.1 Test procedures Withstand voltage tests The voltage shall be applied to the test object starting at a value sufficiently low to prevent any effect-of overvoltages due to switching transients. It should be raised sufficiently slowly to permit reading of the measuring instrument but not so slowly as to cause unnecessary prolongation of the stressing of the test object near to the test voltage U. These requirements are in general met if the rate of .rise is about 2% of U per second, when the applied voltage is above 75% of U. It shall be maintained for the specified time and then rapidly decreased, but not suddenly interrupted as this may generate switching transients which could cause damage or erratic test results. The test duration shall be specified by the relevant Technical Committee and shall be independent of the frequency in the range from 45 to 65 Hz. If not specified by the relevant Technical Committee the duration of a withstand test shall be 60 s. The requirements 17.2 of the test are satisfied if no disruptive discharge occurs on tbe test object. Disruptive discharge voltage tests The voltage shall be applied and raised continuously until a disruptive discharge occurs on the test object. The value of the test voltage reached at the instant of the disruptive discharge shall be recorded. The relevant Technical Committee shall specify the rate of rise of the voltage, the number of voltage applications and the procedure for evaluating the test results (see Appendix A). 24 IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) 17.3 Assured disruptive discharge voltage tests The voltage shall be applied and raised continuously until a disruptive discharge occurs on the test object. The value of the test voltage reached at the instant of the disruptive discharge shall be recorded. The requirements of the test are generally satisfied if this voltage is not higher than the assured disruptive discharge voltage on each one of a specified number of voltage applications. The relevant Technical Committee shall specify the number of voltage applications and the rate of rise of the voltage. 25 IS 2071( Part 1) : 1993 IEC Pub 60-l (1989) Section 6: Tests with Lightning Tmpulse Voltage 18 18.1 Definitions for lightning impulse tests Definitions of general applicability These definitions apply to impulses without oscillations or overshoot or to the mean curve drawn through the oscillations and overshoot. 18.1.1 Full lightning impulse A full lightning impulse is a lightning impulse which is not interrupted by a disruptive discharge (see figure 6). See Clause 3 for definition of impulse and 3.1 for distinction between lightning and switching impulses. 18.1.2 Chopped lightning impulse A chopped lightning impulse is a lightning impulse during which a disruptive discharge causes a rapid collapse of the voltage, practically to zero value (see figures 7-9). The collapse can occur on the front, at the peak or on the tail. NOTE - The chopping can be accomplished by an external chopping gap or may occur due to a discharge in the internal or external insulation of a test object. 18.1.3 Value of the test voltage For a lightning impulse without oscillations, the value of the test voltage is its peak value. The determination of the peak value in the case of oscillations or overshoot on standard lightning im$ses is considered in 19.2. For other impulse shapes (see for example figures 10 e-h) the relevant Technical Committee shall define the value of the test voltage taking into account the type of test and test object. 18.1.4 Front time T, The front time TI of a lighming impulse is a virtual parameter defined as I ,67 times the interval T between the instants when the impulse is 30% and 90% of the peak value (points A and B, figures 69). 18.1.5 Virtual origin 0, The virtual origin 0, of a lighming impulse is the instant preceding that corresponding to point A (see figures 69) by a time-0,3T,. For records having linear time scales, this is the intersection with the time axis of a straight line &awn through the reference points A and B on the front. 26 IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) 18.1 A Time to half-value T2 The time to half-value T2 of a lightning impulse is a virtual parameter defined as the time interval between the virtual origin Or and the instant when the voltage has decreased to half the peak value. 18.2 Definitions applicable only to chopped impulses A chopped lightning impulse is a lightning impulse during which a disruptive discharge causes a rapid collapse of the voltage, which then falls to zero or nearly to zero, wjth or without oscillations (see figures 7-9). NOTE - With some test objects or test arrangements, there may be a flattening of the peak or a rounding off of the voltagebefore the final voltage collapse. Similar effects may also be observed due to the imperfections of the measuring system. Exact determination of the parameters related to chopping (18.2.1 to 18.2.5) requires the presence of both a sharp discontinuity and a special measuring system. Other cases are left to the relevant Technical Committees for consideration. 18.2.1 Instant of chopping The instant of chopping is that at which the rapid collapse of voltage which characterizes first occurs. 18.2.2 Time to chopping T, the chopping The time to chopping T, is a virtual parameter defined as the time interval between the virtual origin 0, and the instant of chopping. 18.2.3 Characteristics related to the voltage collapse during chopping The virtual characteristics and D at 70% and 10% of collapse is 1,67 times the the ratio of the voltage at of the voltage collapse during chopping are defined in terms of two points C the voltage at the instant of chopping, see figure 7. The duration of the voltage time interval between points C and D. The steepness of the voltage collapse is the instant of chopping to the duration of voltage collapse. NOTE - The use of points C and D is for definition purposes only; it is not implied that the duration and steepness of chopping can be measured with any degree of accuracy using conventional measuring systems. 18.2.4 Linearly rising front-chopped impulses A voltage rising with approximately constant steepness, until it is chopped by a disruptive discharge, is described as a linearly rising front-chopped impulse. To define such an impulse, the best fitting straight line is drawn through the part of the front between 30% and 90% of the peak amplitude; the intersections of this with the 30% and 90% amplitudes then behg designated E and F. respectively (see figure 9). The impulse is defined by: - the peak voltage U, the front time T,. - 27 IS 2071 ( Pat-t 1) : 1993 IEC Pub 60-l (1989) - the virtual steepness S: S=UIT, This is the slope of the straight line drawn through the points E and F, usually expressed in kilovolts per microsecond. This chopped impulse is considered to be approximately linearly rising if the front, from 30% amplitude up to the instant of chopping, is entirely enclosed between two lines parallel to the line EF, but displaced from it in time by Ito, T, (see figure 9). NOTE - The value and the tolerance on the virtual steepness S shall be specified by the relevant Technical Committee. 18.3 18.3.1 Voltageltime curves Voltageltime curves for linearly rising impulses The voltage/time curve for impulses with fronts rising linearly is the curve relating the peak voltage to the front time TI. The curve is obtained by applying impulses with linear fronts of different steepness. 18.3.2 Voltageitime curve for impulses of constant prospective shape The voltage/time curve for impulses with constant prospective shape is the curve relating the discharge voltage of the test object to the time to chopping, which may occur on the front, at the peak or on the tail. The curve is obtained by applying impulse voltages of constant shape but with different prospective peak values (see figure 11). 19 19.1 Test Voltage Standard lightning impulse The standard lightning impulse is a full lightning impulse having a front time of 1.2 l.ts and a time to half-value of 50 p.s. It is described as a 1.2/50 impulse. 19.2 Tolerances If not otherwise specified by the relevant Technical Committee, the follbwing differences between specified values for the standard inipulse and those actually recorded: Peak value are accepted Front time Time to half-value SO'% SO% NOTE 1 - It is emphasized that the tolerances on the peak value, front time and time to half-value constitute the permitted differences between specified values and those actually recorded by measurements. These differences should be distinguished from measuring errors which are the difference between the values actually recorded and the true values. For information on measuring errors, see IEC publication 60-3 and 60-4. 28 IS 2071 (Part 1) : lYY3 IEC Pub 60-l (1989) With some test circuits, oscillations or an overshoot may occur at the peak of the impulse, see figures 10 a to d; if the frequency of such oscillations is not less than 0.5 MHz or the duration of overshoot not more than 1 p, a mean curve should be drawn as in figures 10 a and b and, for the purpose of measurement, the maximum amplitude of this curve is chosen as the peak value defining the value of the test voltage. Overshoot or oscillations in the neighborhood of the peak, measured by a system according to IEC Publication 60-3, are tolerated provided their single peak amplitude is not larger than 5% of the peak value. In commonly used impulse generator circuits, oscillations on that part of the wavefront during which the voltage does not exceed 90% of the peak value have generally negligible influence on test results. If the relevant Technical Committee finds these are of importance, it is recommended that their amplitudes, measured by a suitable measuring device, as specified in IEC Publication 60-3, are under the straight line drawn through the points A' B' (see figure 12). These points are taken on the verticals of, respectively, the points A and B determLzd according to 18.1.4, the distance AA' being equal to 25% and BB' to 5% of the peak value. The impulse should be essentially unidirectional, but see Note 2. NOTE 2 - In specific cases. such as during tests on low impedance objects or on UHV test circuits having large dimensions, it may be impossible to adjust the shape of the impulse within the tolerances recommended, to keep the oscillations and/or the overshoot within the specified limits or to avoid a polarity reversal. Such cases, should be dealt with by the relevant Technical Committee. 19.3 Standard chopped lightning impulse A standard chopped lightning impulse is a standard impulse chopped by an external gap after 2 to 5 p. Other times to chopping may be specified by the relevant Technical Committee. Because of practical difficulties in measurements, the duration of voltage collapse has not been standardized. 19.4 Special lightning impulses In some cases oscillating lightning impulses may be applied. This offers the possibility of producing impulses with shorter front times or with peak values corresponding to a generator efficiency greater than 1. 19.5 Generation of the test voltage The impulse is usually produced by an impulse generator consisting essentially of a number of capacitors which are charged in parallel from a direct voltage source and then discharged in series into a circuit which includes the test object. 19.6 Measurement Measurement of the test voltage and determination of impulse shape with devices approved under IEC Publication 60-3 19.6.1 The measurement of the peak value, the time parameters and the overshoot or oscillations on the test voltage should in general be made with devices which have passed the approval procedure referred to in IEC Publication 60-3. The measurement shall be made with the test object in the circuit and, in general, the impulse shape shall be checked for eachtest object. Where a number of test objects of the same design and size are tested under identical conditions, the shape needs only to be verified once. 29 IS 2071(Part I) : 1993 IEC Put> 6O;l (1989) NOTE - Determination of the impulse shape by calculation from the test circuit parameters is not considered to be satisfactory. 19.6.2 Calibration of a non-approved measuring device with an approved measuring ,device The procedure usually consists of establishing a relationship between the display of some device related to the test voltage (for instance the maximum charging voltage of the first stage of the impulse generator) and a measurement of the same voltage performed in accordance with 19.6.1 or with a sphere-gap, used in accordance with LEC'Publication 52. The relationship may be dependent on the presence of the test object, of the sphere-gap, etc. Hence, it is important that these conditions are the same during the calibration and the actual test, except that during the test the sphere-gap may be opened sufficiently to prevent sparkover. For tests on objects with self-restoring insulation, the calibration should be made at or near 100% of the test voltage. For tests on objects with non-self-restoririg insulation, extrapolation may be unavoidable but such extrapolation shall be made from not less than 50% of the test voltage. The extrapolation is only permissible if it can be shown that the test voltage is proportional to the related quantity. 19.7 Measurement of current during tests with impulse voltages The relevant Technical Committee shall specify the characteristics of a current flowing in the test object that should be measured during tests with high impulse voltages. When this type of measurement is used for comparative purposes wave shape is of importance and the measurement of the absolute value of this current may be of lesser importance. 20 20.1 Test Procedures Withstand voltage tests The recommended test procedure depends on the nature of the test object, as defined in Clause 5. The relevant Technical Committee shall specify which procedure shall be applied. In procedures A. B and C the voltage applied to the test object is only the specified withstand value, while in procedure D several voltage levels have to be applied. 20.1.1 Withstand voltage test: Procedure A Three impulses of the specified shape and polarity at the rated withstand voltage level are applied to the test object. The requirements of the test are satisfied if no indication of failure is obtained, using methods of detection specified by the relevant Technical Committee. NOTE - This procedure is recommended for tests on degradable or non-self-restoring insulation. 30 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) 20.1.2 Withstand voltage test: Procedure B Fifteen impulses of the specified shape and polarity at the withstand voltage level are applied to the test object. The requirements of the test are satisfied if not more than two disruptive discharges occur in the self-restoring part of the insulation and if no indication of failure in the non-self-restoring insulation is obtained by the detection methods specified by the relevant Technical Committee. 20.1.3 Withstand voltage rest: Procedure C Three impulses of the specified shape and polarity ai the withstand voltage level are applied to the test object. If no disruptive discharge occurs the test object has passed the test. If more than one disruptive discharge occurs the test object has failed to pass the test. If one disruptive discharge occurs in the self-restoring part of the insulation, then nine additional impulses are applied and if no disruptive discharge occurs the test object has passed the test. If any detection of failure in a non-self-restoring part of insulation is observed with the detection methods specified by the relevant Technical Committee during any part of the test, the test object has failed to pass the test. NOTE - This procedure corresponds to an American practice modified so as to be statistically equivalent to Procedure B . 20.1.4 Withstand voltage test: Procedure D For self-restoring insulation the 10% impulse disruptive discharge voltage U10may be evaluated by using statistical test procedures described in Appendix A. These test methods permit either direct evaluation of U,, and U,, or indirect evaluation of Ul,, In the latter case U10is derived from the U,, value using the relationship: &J= &,(I - 1,32) The relevant Technical Committee shall specify the value to be assumed for the conventional deviation z of the disruptive discharge voltage. For dry tests on air insulation, without any other insulation involved, the per-unit value z = 0,03 can be used. The test object is deemed to be satisfactory if UIOis not less than the specified impulse withstand voltage. The following test methods can be used to evaluate UsO: a) the multiple-level per level; method (see Clause A.l.l) with n 24 voltage levels, and m 2 10 impulses b) the up-and-down method (Clause A.1.2) with m = 1 impulse per group and n 2 20 useful applications. To evaluate U,, , the up-and-down withstand method, with m = 7 impulses per group and at least eight useful groups, can be used. 31 IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) In all the cases the voltage interval between levels AU should be approximately estimated value of U,, . 20.2 Procedures for assured discharge voltage tests CO from 1,5 to 3% of, the The procedures for an assured discharge voltage test are similar appropriate changes between discharge and withstand conditions. those described in 20.1 with the The relevant Technical Committee may also specify other procedures for specific test objects. 32 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) Section 7: Tests with Switching Impulses 21 21.1 Definitions for switching impulse tests Switching impulse of a A switching impulse (as distinct from a lightning impulse) is defined in 3.1. The characteristics switching impulse are expressed by the parameters defined in 21.2 to 21.7 (see figure 13). Additional parameters can be specified by the relevant Technical Committee when considering tests. 21.2 Value of the tesl voltage by the relevant Technical Committee, specific If not otherwise specified prospective peak value. 21.3 Time IO peak T, the value of the test voltage is the The time,to peak TP is the time interval between the actual origin and the instant when the voltage has reached its peak value. 21.4 Time to half-value T, The time to half-value T2 for a switching impulse is the time interval between the actual origin and the instant when the voltage has first decreased to half the peak value. 21.5 Time above 90% Td The time above 90% T,, is the time interval during which the impulse voltage exceeds 90% of its peak value. ` 21.6 Time to zero To The time to zero To is the time interval between the actual origin and the instant when the voltage has its first passage to zero. Specification of the time above 90% and time to zero instead of the time to half-value is found useful, for instance, when the form of the impulse is dictated by saturation phenomena in the test object or the test circuit, or where the severity of the test on important parts of internal insulation of the test object is considered to be highly dependent on these parameters. When specifying a switching impulse, only one set of parameters related ti the waveshape is generally given. The particular time parameters defined should be clearly indicated by reference, for example, to a TPI T2 or TP/ Td / T,, impulse. The front duration for switching impulses is sometimes alternatively defined in the same manner as the front for lightning impulses (18.1.4) or in a similar marmer with other reference points and multiplying factors. For switching impulses with time parameters as given in 22.1. the time to peak is between 1.4 and 1,8 times the front time. NOTE - to 21.3 to'21.6 - 33 IS 2071 (Part I) : 1993 IEC Pub 60-l (1989) 21.7 Time to chopping T, The time to chopping T, of a switching impulse is the time interval between the actual origin and the instant of chopping. 21.8 Linearly rising impulse The definition of a linearly rising impulse (applicable to both lightning and switching impulses) is given in 18.2.4. 22 22.1 Test voltage Standard switching impulse T2 of 2500 p. The standard switching impulse is an impulse having a time to peak T, of 250 p and a time to half-value It is described as a 250/2500 impulse. Tolerances 22.2 If not otherwise specified by the relevant Technical Committee, the following differences are accepted between specified values and those actually recorded, both for standard and special impulses (see Note 1 to 19.2): Peak value Time to peak Time to half-value +30/o S!O% +60% In certain cases, for instance with low impedance test objects, it may be difficult to adjust the shape of the impulse to within the tolerances recommended. In such cases other tolerances or other impulse shapes may be specified by the relevant Technical Committee. NOTE - The disruptive discharge voltage of long gaps in air may be influenced by both the time to peak and the time to half-value of a switching impulse. Therefore it is recommended for such test objects that the applied switching impulse be characterized by its actual time parameters. Larger tolerances in the prospective time to half-value may be alkwed in the case of a disruptive discharge occurring before or at the peak. 22.3 Special switching impulses For special purposes, when the use of the standard switching impulse is not considered sufficient or appropriate, special switching impulses of either aperiodic or oscillating form may be prescribed by the relevant Technical Committee. electrode. two impulses NOTE - When a discharge is initiated by a leader in air from a positively-charged may generally be considered as equivalent, when they have the same peak value and the same time interval between the respective two points on the front at 70% and 100% of the peak value. 34 IS 2071 (Pat-t 1) : 1993 IEC Pub 60-l (1989) 22.4 Generation of the test voltage Switching impulses are usually generated by a conventional impulse generator (see 19.5). They can also be generated by the application of a voltage impulse to the low-voltage winding of a testing transformer (or of a transformer to be tested). Other methods of generating switching impulses can be used, for example, involving the rapid interruption of current in a transformer winding. The elements of a circuit for generating switching impulses should be chosen so as to avoid excessive distortion of the impulse shape due to non-disruptive discharge currents in the test object. Such currents can reach quite large values, especially during pollution tests on external insulation at high voltages. In test circuits having high internal impedance, they may cause severe distortion of the voltage or even prevent a disruptive discharge from occurring. 22.5 Measurement of test voltage and determination of impulse shape The measurement of the test voltage and the determination of the impulse shape should'be made as described in 19.6.1 and 19.6.2. Note that although IEC Publication 52, 1960, gives no information specifically related to the measurement of the peak value of switching impulses, measurements indicate that the sphere-gap can be regarded as an approved measuring device for switching impulse voltages. 23 Test procedures The test procedures are in general the same as for lighming impulse testing and similar statistical considerations apply (see Clause 20 and Appendix A). Unless otherwise specified by the relevant Technical Committee, the conventional deviation of the disruptive discharge voltage for dry and wet tests on air insulation, without any other insulation involved. can be assumed to be: 2 = 0.06 Correspondingly larger voltage intervals AU may be used when applying the multiple level or the up-anddown procedures. NOTE - With switching impulses, disruptive discharges frequently occur at random times well before the peak. In presenting the results of discharge tests,made in accordance with 20.1.4. the relationship between disruptive discharge probability and voltage is generally expressed in terms of the prospective peak value. However, another method is also in use in which the actual disruptive discharge voltage for every impulse is measured; the probability distribution of the measured voltage values is then determined by the method. described for Class 3 tests in Appendix A. 35 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) Section 8: Tests with Impulse Current 24 24.1 Definitions for impulse current tests Impulse current Two types of impulse currents are used. The first type has a shape which increases from zero to peak value in a short time, and thereafter decreases to zero either approximately exponentially or in the manner of a heavily-damped sine curve. This type is defined by the front time T1 and the time to half-value T, (see 24.3 and 24.5). The second type has an approximately the total duration (see 24.6 and 24.7). 24.2 Value of the test current rectangular shape and is defined by the duration of the peak and The value of the test current is normally defined by the peak value. With some test circuits, overshoot or oscillations may be present on the current waveform. The relevant Technical Committee shall specify whether the value of the test current should be defined by the actual peak or by a smooth curve drawn through. the oscillations. 24.3 Front time T, The front time T, of an impulse current is a virtual parameter defined as 1.25 times the interval T, between the instants when the impulse is 10% and 90% of the peak value (see figure 14a). If oscillations are present on the front, the 10% and 90% values shall be derived from a mean curve drawn through these oscillations in a manner analogous to that used for lightning impulses with oscillations on the front. 24.4 Virtual origin O1 The virtual origin 0, of an impulse current precedes by O.lT, that instant at which the current attains 10% of its peak value. For records having linear time scales, this is the intersection with the time axis of a straight line drawn through the 10% and 90% reference points on the front. 24.5 Time to half-value T2 The time to half-value T2 of an impulse current is a virtual parameter defined as the time interval between the virtual origin 0, and the instant at which the current has decreased to half the peak value. 24.6 Duration of peak of a rectangular impulse current Td The duration of the peak of a rectangular impulse current Td is a virtual parameter defined as the time during which the current is greater than 90% of its peak value (see figure 14b). IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 24.7 Total duration of a rectangular impulse current T, The total duration of a rectangular impulse current T, is a virtual parameter defined as the time during which the current is greater than 10% of its peak value (see figure 14b). If oscillations are present on the front, a mean curve should be drawn in order to determine the time at which the 10% value is reached. 25 25.1 Test current Standard impulse currents Four standard impulse currents corresponding to the first type of impulse, defined in 24.1, are used. - l/20 impulse: 4/10 impulse: 8/20 impulse: 30/80 impulse: front time: front time: front time: front time: 1 ps; 4 ps; 8 ps; 30 Ps; time to half-value: time to half-value: time to half-value: time to half-value: 20 P-s; IO CLS: 20 Iis; 80 W; Rectangular impulse currents have duration of the peak Td of 500 ps, 1000 l.ts or 2000 l.ts or between 2000 l.ts and 3200 l.rs. 25.2 Tolerances If not otherwise specified by the relevant Technical Committee, the following differences between the specified values for standard impulse currents and those actually recorded: For l/20,4/10, 8/20 and 30/80 impulses: f 10% f f are accepted peak value front time TI time to half-value T, 10% 10% A small overshoot or oscillations are tolerated provided that their single peak amplitude in the neighbourhood of the peak of the impulse is not more than 5% of the peak value. Any polarity reversal after the current has fallen to zero shall not be more than 20% of the peak value. For rectangular impulses: peak value duration of the peak +20%; +20%; -0% -0% 37 IS 2071( Part 1) : 1993 IEC Pub 60-l (1989) An overshoot or oscillations are tolerated provided that their single peak amplitude is not more than 10% of the peak value. The total duration of a rectangular impulse shall not be larger than 1.5 times the duration of the peak and the polarity reversal should be limited to 10% of the peak value. 25.3 Measurement of the test current The test current shall be measured by a device which has passed the approval procedure referred to in IEC Publication 60-3. 25.4 Measurement of voltage during tests with impulse current Voltages developed across the test object during tests with high impulse currents should be measured by a device which has passed the approval procedure given in IEC Publication 60-3 for the measurement of impulse voltages. NOTE - The impulse current may induce appreciable voltages in the voltage measuring circuit, causing significant errors. As a check, it is therefore recommended that the lead that normally joins the voltage divider to the live end of the test object should be disconnected from this point and connected instead to the earthed end of the test object, but maintaining approximately the same loop. Alternatively, the test object may be short-circuited or replaced by a solid metal conductor. The test circuit geometry should be modified until the voltage measured when the generator is discharged under any of these conditions is negligible in comparison with the voltage across the test object, at least during the part of the impulse which is of importance for evaluating the test results. 38 IS 2071 (Pax-t 1) : 1993 IEC Pub 60-l (1989) Section 9: Combined and Composite Tests 26 Combined voltage tests A combined voltage test is one in which two separate sources, generating voltages against earth, are connected to two terminals of the test object, (for example an open circuit breaker, see figure Isa). In such a test any two of lightning impulse, switching impulse, direct or power frequency alternating voltages may be combined. The test voltage is characterized polarity of each component. by its amplitude, a time delay At and by the waveshape, peak value and When combined voltage tests are performed on switchgear they are intended to simulate conditions where one terminal of the open switch is energized at the specified power frequency voltage, and the other terminal is subjected to either a lightning or switching overvoltage. The test circuit shall simulate thus situation on both internal and external insulation. In special cases the relevant Technical Committee may permit power-frequency voltages to be simulated by switching impulses of suitable shape. 26.1 Value of the test voltage U The value of the test voltage U is the maximum potential difference between the energized terminals of the test object (see figure 15b). 26.2 Time delay At The time delay At of a combined voltage is the time interval between the instants when its components reach their peak values, measured from the instant of a negative peak (see figure 20). It has a tolerance where T, is the time to peak or the front time for an impulse and a quarter cycle for an of ti,OST,,,,, alternating voltage, and T,,,, is the larger of the values of TP for the two components. Two voltages of a combined impulse voltage test are said to be synchronous zero, within the prescribed tolerance. 26.3 Actual voltage shapes when their time delay At is Due to the coupling between the two generating systems, the shapes and amplitudes of the two components of a combined voltage test differ from those produced by the same sources used separately. They shall therefore be measured in combination, preferably by means of separate measuring systems against earth. Each measuring system shall be suitable for measuring the waveshape of both of the components in order to avoid errors in recording their mutual influence. The maximum permissible deviations from the prescribed voltage shape shall be specified by the relevant Technical Committee. NOTE - It should be taken into account that in the case of a disruptive discharge occurring in a combined voltage test, both the voltage sources will act directly against each other if there are no additional protective elements (e.g.. resistors or protective gaps) in the circuit. In any case the voltage distribution between the two voltage sources will change completely when there is a disruptive discharge. IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) 26.4 Arrangement of fhe test object with respect to the earthed structures shall be specified The arrangement of the test object, particularly by the relevant Technical Committee. 26.5 Atmospheric correction factors In a combined voltage test, the atmospheric correction factors relative to the component of highest value have to be applied to the test voltage value. 27 Gomposite tests A composite voltage is the voltage resulting from two different voltage sources suitably connected, applied at one terminal of the test object against earth. The definition of its parameters is left to the relevant Technical Committee. NOTE - Composite tests may also be performed by applying voltage object. and impulse-current sources to the test 40 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) Appendix A: Statistical Treatment of Test Results A.1 Classification discharge of tests test procedures can be divided into three classes for the purpose of statistical Disruptive evaluation. A.l. 1 Class 1: Multiple-level tests In a Class 1 test, m; substantially equal voltage stresses (e.g., lightning impulses) are applied at each of n voltage levels Vi (i = 1, 2, .. .. n). While this procedure is usually employed with impulse voltages, some tests with alternating and direct voltages also fall into this class. The test results are the n numbers mi of voltage applications and the corresponding numbers di of disruptive discharges at each voltage level Vi. A.1.2 Class 2: Up-and-down tests In a Class 2 test, n groups of m substantially equal voltage stresses are applied at voltage levels Vi. The voltage level for each succeeding group of stresses is increased or decreased by a small amount AU according to the result of the previous group of stresses. Two testing procedures are commonly used. The withstand procedure, aimed at finding voltage levels corresponding to low disruptive discharge probabilities and the discharge procedure, which finds voltage levels corresponding to high disruptive discharge probabilities. In the withstand procedure, the voltage level is increased by an amount AU if no disruptive discharge occurs in a group of m voltage applications, otherwise the voltage level is decreased by the same amount. In the discharge procedure, the voltage level is increased by AU if one or more withstands occur, otherwise it is decreased by the same amount. Where m = 1. the two procedures become identical and correspond to the up-and-down discharge voltage test. 50% disruptive Tests with other values of m are also used to determine voltages corresponding to other disruptive discharge probabilities. The results are the numbers ki of stress groups applied at the voltage levels Vi. The first level Ui taken into account is that at which at least two groups of stresses were applied. The total number of useful groups is n = C ki. A.1.3 Class 3: Successive Discharge Tests In a Class 3 test, a procedure leading to a disruptive discharge on the test object is applied n times. The test voltage may be increased continuously until a disruptive discharge occurs or held constant at some level until a disruptive discharge is observed. The results are the n values of voltage Vi OF time ti at which the disruptive discharge occurred. Such tests are made with direct, alternating or impulse voltages. Tests where disruptive discharges occur on the front of the impulse fall into this class. 41 IS 2071 ( Part 1) : 1993 IEC Pub 60-l (1989) A.2 Statistical Behaviour of Disruptive Discharge When p, the probability of a disruptive discharge during a given test procedure, depends only on the test voltage, U, the behaviour of the test object can be characterized by a function p(U) determined by the processes of discharge development. In practice, this function, the disruptive discharge probability function, can be represented mathematically by expressions depending on at least two parameters U,, and z. U,, is the 50% discharge voltage for which p(U) = 0,5 and z is the conventional deviation; z = Us,- U,, where Cl16 is the voltage for which p(v) = 0,16 NOTE 1 - Examples of p(v) can be derived from the Gaussian (or Normal), the Weibull or the Gumbel probability distribution functions. Experience shows that for 0,lS c p c 0,85 most theoretical distributions can be considered equivalent. Special Weibuil or Gumbel distributions are acceptable approximations to a Gaussian distribution having given Us0 and z f6r p lying between 0.02 and 0.98. Beyond these limits little information is available. NOTE 2 - Sometimes p is a i-unction of two or more parameters, e.g., V and dU I dt. In such cases no simple function can be used to describe p: Details of such cases may be found in the technical literature. The function p(U) and the parameters Us, and z can be found from tests with very large numbers of voltage applications, provided that the characteristics of the test object remain constant throughout the tests. In practice the number of voltage applications is usually limited and the estimates of an assumed form of p(v) will be subject to statistical uncertainties; A.2.1 Confidence limits and statistical error U, and z based on If a parameter y is estimated from n test results, upper and lower confidence limits yu and ye can be defined, with the probability C that the true value of y is within these limits. C is termed the confidence level and the half width e, = Cyv- yL) / 2 of the confidence band is called the statistical error. Usually C is taken as 0.95 (or 0.90) and the corresponding limits. limits are called the 95% (or 90%) confidence The statistical error e, depends on both n and the value of the conventional deviation z. The conventional deviation z should be estimated when possible from tests made under realistic conditions. In general, the larger the number of tests made, the better will be the estimate of t. It should, however, be remembered that during a protracted test series, ambient conditions may change to an extent which offsets the gain in accuracy from the increased number of tests. Since accurate estimation of z from a limited series of tests is not possible. values estit pooled results of many tests are often given by the relevant Technical Committees. lted from the The statistical error e, may be combined with estimates of other errors (e.g., measuring errors) tc define the overall error limits for the determination of a particular parameter. 42 IS 2071 (fart 1) : 1993 IEC Pub 60-l (1989) A.3 Analysis of Test Results This Clause is applicable to cases where the results of tests can be regarded as independent estimates, i.e., where the nth result is not influenced by what may have occurred in the (n - 1)th or (n -j)th tests. A.3.1 Treatment of Results from Class 1 Tests In this case the discharge frequency f;:= di lmi at a voltage level Vi is taken as an estimate of &Vi) the discharge probability at the voltage level Vi. The n estimates of &Vi) obtained in a Class 1 test can then be fitted to an assumed probability distribution function p(v) and the parameters Us, and z determined. This may be done by plotting8 versus Vi on special graph paper designed to give a straight line plot when the probability estimates conform to a particular probability distribution function p(v). A well-known example is Gaussian or Normal probability paper which yields a straight line plot for estimates conforming to the Gaussian distribution function: p(v) = (l/z a) J", exp [- (v - U&`/22 `1 du NOTE - Normal probability papers do not have ordinate scales embracing the values p = 0 or p = 1. Accordingly, tesfs at voltage levels causing all discharges di = m; or no discharges C$= 0 cannot be plotted directly. A possible way of using these results is to combine to plot them as the weighted mean voltage. them with values obtained for an adjacent voltage level and Alternatively analytical fitting techniques involving the least-squares method or likelihood methods (see A.4) may be used to find UsD,L and the confidence limits of these estimates. In any case adequate methods (such as conventional regression coefficients or confidence limits) should be used to check if the assumed probability function fits the measured points with sufficient accuracy. Reference is made to the relevant technical literature. As a general guide the statistical error tends to vary inversely as the square root of the number of voltage applications at each level mi and inversely as the number of levels used n; Note also that if all values of fi differ from zero and unity, with 10 voltage applications (m = 10) at each of five levels (n = 5) the 95% confidence limits would be: For US0: (U:, - 0.75 z*) I u,, 5 2071 (Part 1) : 1993 Pub 60-l (1989) 3/6 standard pipe thread 1 10 v ---u-u- 5/32 l/16 0.0575 0.0674 0.1111 Dimensions In inchos Figure 2 d) Nozzle type IV (American practice). NOTE -.The to 3 m. nozzle type IV in Figure Id (for American practice) has a concentric orifice with dimensions given in the figure. With water pressure of 250 kPa - 450 kPa it gives jet lengths ?f 2 m IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) 12 I I l Alternating k _._._.B voltage Impulse -w-w- voltage Direct voltage 191 Voltage Impulse 1 1+0,010 1+0,012 k Humidity Range, <15 <15 <13 g/m3 Alternating I Direct 0.8 1 1+0,014 I 0 5 For valuer of h/d -15% may occur. In excess of 15 g/m3 l rrom as much OS 10 15 20 (g/m31 25 30 35 Figure 3 - C as a function of the ratio of absolute humidity h to the relative air density 6. 56 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 1.0 m W 0.5 Figure 4 Values of exponents m for air density correct&n and w for humidity correction as a function of parameter g: see 11.2.3 NOTE - The wlues of exponents m md w $we been deduced from expairnaM vducs obwincd in different conditions. However, they are limited to &itudes &we tbc LU level of less thrn 2000 m. 57 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) 80% 25 60% 10 5 0 0 5 Ambient 10 15 - 20 dry-bulb 25 30 temperature 35 OC temperature Absolute humidity of air as a function of dry- and wet-bulb thermometer readings; see 11.5. Curves of relative humidity are also given. 58 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) U 1.0 0.9 0.5 (r.3 0 i ------7 Tl = 1.67T T`= 0.3T, = 0.5T T' - Figure 6 Full lightning impulse. 1 1 .( O.! --_--------- 0`., 0.2 0.1 c 01IT, Figure 7 Lighming impulse chopped on the front. . 59 -- -. a --. O"' IFigure 8 /I T, . Lightning impulse chopped on the tail. I I I I I I I I I I I I I I I I I Figure 9 _ Linearly rising front chopped impulse. 60 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) ll!zz E,) f 2 0.5MHz 0 1 2 3 4 f(p) 0 1 2 3 0) e f <0.5MHZ b) E 0 1 2 c) 3 4 fo1s) 4 0.2 4 6 6 tips) 0 2 4 6 f) 6 4 0 I I , I I I I I I c YIJd 0 2 4 6 h) 6 f(lJs) . 2 4 6 6 a) Figure l0 Examples of lightning impulses with oscillations or overshoot. a.b The value of the test voltage is determined by a mean curve (broken line). c.d The value of the test voltage is determined by the crest value. e,fg,h No general guidance can be given for the determination of the value of the test voltage. 61 IS 2071(l'art1):1993 IEC Pub 60-l(1989) Figure 11 Voltage/time curve for impulses of constant prospective shape. IS 2071 (Rut 1) : 1993 IEC I'ub 60-l (1989) -V t ,.o() ___________" -----------_ 0.95 Figure 12 Waxinium permissible amplitude of OsciIIationson the wave-front". Figure 13 Full switching impulse. 63 IS 2071( Part 1) : 1993 XEC Pub 60-l (1989) 190 099 .---_----_-_ 8 -- 095 \ -c------ 091 0 I-- T,-? I T2 L 1 Figwe 14 a) Impulse current- Exponentiti. l,O 03 OS1 0 Figure 14 b) Impulse current- Rectangular. 64 IS 2071(l'ort1):1993 IEC Pub 60-l(1989) Test transformer ----- r IW I I Impulse -------- generator 1 I I I /`OI I I I I f I L -------I I I _ -I I Figure 15 a) Example of test circuit for combined voltage tests. Figure 15.b) Example of voltage waves during combined voltage tests giving value of test voltage U. 65 IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) 1 .o I I I 0.1 iiiiittti I 11111111 I I I Illffl r;----' I I --J ---- c 0.01 0.1 1.0 Solinlty 10 100 1000 g/L Figure Ifi Resistivity of a solution of sodium chloride (NaCl) in water, as a function of salinity at solution temperature I of lo'C, w)`C, and 30'C. NOTE The tdnity is expressed in grams pa litre as detcmkmd at a tempcmture of 2o'C. 66 IS 2071( Part 1) : 1993 IEC Pui 60-l (1989) 1.15 1.10 f 0 s 1.05 1.00 0 5 100 Salinty 150 200 250 g/L Figure Ii Density of solution of sodium chloride (NaCl) in water. as a function of salinity at solution temperaturef of WC, 2O'C.and 3o'C. NOTE - The salinity ,ir expressed in grsms per hare as detemined at a temperature of WC. 67 ls 2071 (Part 1) : 1993 IEC Pub 60-l (1989) Square-cut and polished \- d' I 3 5~ 0.05 3 f f 0.05 0.02 ,": `: 1.2 d' B 2 * 0.02 Clamping ha Dimensions in millimetres A=perspexbody B=mndardcouplingfor8mmnomindboretube (stainless steel) C 0 sknless steel (6 mm nominal SI thread with 1.6 mm bore tube) D = nylon (6 mm nominal SI thread, 16 mm long saew with concentric ruinless steel tube) E = perspex plug Figure 18 Saline fog jet; see Appendix A. 68 IS 2071 (Part 1) : 1993 IEC Pub 60-l (1989) i I 200f0700 d 250 d d d 2500 Figure 19 a) Vertical arrangementof rod/rodgap. I 2700 >lOOO - p 2506 d62500 >lOOO t 300 @ i i i i i i I I i i i i i i i i i i ! I Irolatour lns~lator 7 gap. 69 2 4000 Figtm l.9 b) &Ihizmt8I arrangementof rod/d IS 2071(Part 1) : 1993 IEC Pub 60-l (1989) u I t- b) Figure 2Q Definition of time de@ AL a) Combination of two impulse voltages. b) CombAtion of an impulse voltage `And a power frequencyaltanating voltage. 70 IS 2071 ( Part 1 ) : 1993 IEC Pub 60-l ( 1989 ) NATIONAL ANNEX ( National Pigure 1 given in the main text is to be replaced Foreword ) by the following figure. 2 P 5'0 !/ - _ --- 3 2-s 4 I I750 0 5al I 1 loo0 I 500 2000 U (kV) peak Pigure 1 Minimum clearance D of extraneous live or grounded objects to the energized electrode of a text object during an ac or positive switchiilg impulse test at the maxiqum voltage U applied during test 71 Bureau of Indian Standards BIS is a statutory institution established under the Bcmnu ofhndian Standards Act, I986 to promote harmonious development of the activities of standardization, marking and quality certification of goods and attending to connected matters in the country. 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